Ammonium metallates can as a rule be readily crystallized and can be converted into metal oxides by thermal treatment. These can be reduced to the corresponding metals. In particular, ammonium metallates of molybdenum, for example ammonium molybdate, ammonium dimolybdate and ammonium polymolybdate, but also other oxo anion-forming metals, such as, for example, tungsten, rhenium, vanadium, niobium or tantalum, are therefore valuable compounds for obtaining corresponding metal oxides or metals. For a number of applications, it is decisive that the metals or metal compounds used have as low a content of impurities as possible. The recovery of pure molybdenum or pure molybdenum compounds is effected as a rule starting from molybdenite concentrates by oxidation, usually roasting of the sulphide in air, to give industrial molybdenum trioxide. This contains a whole range of impurities which, depending on origin and time of extraction of the corresponding molybdenite ore, may vary greatly in type and concentration. For the various uses of molybdenum, for example as an alloy component in steels, for wire production, as a catalyst component or as a pigment, the purity of the molybdenum metal or of the molybdenum compound has to meet very different requirements. A number of different processes for the working-up and purification of molybdenum or the compounds thereof, which utilize, for example, the principles of precipitation/crystallization, extraction or ion exchange, have therefore been developed. Impurities, in particular alkali metals, can be particularly effectively removed by sublimation of the roasted materials. However, this method is very complicated and is not practicable economically on an industrial scale.
Of course, not all impurities behave in the same way, both concerning the tolerable amounts in the product and concerning the separability. In particular, the element potassium can be separated from the molybdenum only with considerable effort. The potassium content must as a rule be <50–100 ppm in order to produce molybdenum powders which are suitable for the production of wires, rods, foils, sintered parts and especially alloys. The use of molybdenum in the area of catalysts or pigments also often requires chemically pure, in particular low-potassium molybdenum salts.
U.S. Pat. No. 4,612,172 describes a process for removing potassium from MoO3 by leaching with a mixture of mineral acid and ammonium salt of the mineral acid, the mixture being said to be used in a three-fold excess relative to the molybdenum oxide. This process is also utilized in EP 191 148 A2, leaching being effected repeatedly here in order to reduce the potassium content to a value of about 50 ppm. According to U.S. Pat. No. 4,596,701, U.S. Pat. No. 4,525,331, U.S. Pat. No. 4,604,266 and U.S. Pat. No. 4,601,890, the purification of MoO3 by leaching with a mixture of mineral acid and the ammonium salt of a mineral acid can be optimized if, after the leaching, the molybdenum trioxide is reacted with ammonium hydroxide to give ammonium molybdate, the ammonium molybdate solution is brought into contact with a cation exchanger and finally the ammonium molybdate thus purified is crystallized and calcined. The process differs substantially in the choice of the acid/ammonium salt mixture. Said processes are comparatively complicated, multistage processes which require the use of large amounts of leaching agent and have the further disadvantage that molybdenum fractions which go into solution together with the impurities during the leaching are not recovered.
In order to avoid the loss of molybdenum through the leaching, U.S. Pat. No. 4,933,152 proposes a process in which a first leaching with nitric acid/ammonium nitrate and a second leaching with nitric acid are effected, the mother liquors being recycled. In this way, the process can be carried out continuously and the molybdenum fractions remain in the system. Nevertheless, this process too is associated with the major disadvantage that technically complicated leaching/filtration reactions have to be carried out repeatedly and large amounts of nitric acid-containing solutions have to be handled and circulated.
According to U.S. Pat. No. 3,848,049, impurities can be removed from a roasted molybdenum oxide concentrate also by leaching with water, the leaching optionally being carried out in the presence of a cation exchange resin. The residue is converted by addition of ammonium hydroxide into ammonium molybdate, which is then crystallized and can be further processed. The content of potassium in the product is stated as 150 ppm in example 2 and is thus still above the value which can be tolerated for many applications. In order to increase the separability of potassium, U.S. Pat. No. 3,393,971 proposes a particular thermal treatment of the molybdenum oxide. According to U.S. Pat. No. 4,207,296, too, the solubility of potassium in hot water is said to be increased by subjecting the molybdenum oxide to a thermal treatment. Heating is effected to at least 525° C. and cooling is then effected at a rate of at least 30° C. per minute to a temperature of less than 400° C. Corresponding thermal treatments are complicated in terms of apparatus and instrumentation and as a rule do not lead to reproducible residual contents of potassium. Such a procedure might be of technical interest only for those processors of molybdenum ores which carry out roasting of molybdenite themselves. For the further purification of industrial molybdenum oxide and other molybdenum compounds, such a procedure is unattractive, inter alia owing to the high energy consumption.
Another concept for removing impurities, in particular potassium, from molybdenum and compounds thereof is solvent extraction. Here, sodium molybdate is first transferred into an organic phase by acid extraction, the impurities remaining in the refined product. The molybdate is then removed again from the organic phase in the form of ammonium molybdate by stripping with ammonia. It is thus possible to obtain very pure ammonium molybdate solutions, but the process is very complicated and expensive. In particular, the fact that large amounts of organic solvent have to be used is disadvantageous.